From the perspective of energy conservation, IGBTs and diodes are used in power modules and the like for performing variable speed control of three-phase motors in the fields of general-purpose inverters, AC servo motors, and the like. As an IGBT or a diode, a device with low switching, loss and low on-voltage is desired in order to reduce inverter loss.
Resistance of a thick n-type base layer necessary for withstand voltage retention is responsible for most of an on-voltage, and the resistance is effectively reduced by thinning a wafer. However, in the case of a thinned wafer, a depletion layer reaches a back surface when voltage is applied to a collector and a decline in withstand voltage or an increase in leakage current occurs. Therefore, generally, a shallow n+-type buffer layer with a higher concentration than a substrate concentration is formed on a back surface of a substrate by an ion implanter.
However, due to reductions in wafer thickness down to a vicinity of a thickness where withstand voltage can be secured in accordance with advances in IGBT manufacturing techniques, with a shallow n+-type buffer layer, when a surge voltage determined by power supply voltage +L*di/dt is applied between a collector and an emitter or between a cathode and an anode during a itching operation of an IGBT or a diode and a depletion layer reaches a back surface side, a carrier becomes depleted and oscillations of voltage and current occur. An occurrence of oscillations generates radiation noise which adversely affects peripheral electronic devices.
On the other hand, by forming a deep n+-type buffer layer of around 30 μm with a low concentration on a substrate back surface, even when a large voltage is applied to a collector or a cathode during switching, a depletion layer can be gradually stopped. As a result, by preventing a carrier on the back surface side from becoming depleted and causing the carrier to stay on the back surface side, an abrupt rise in voltage can be prevented.
FIG. 23 is a diagram showing a turn-off waveform of L load switching performed in a device simulation using an IGBT with a withstand voltage of around 1200 V. Switching conditions include: n+-type buffer layers formed of phosphorus with depths of 2 μm and 30 μm; Vce=900 V; and Ic=150 A. While the waveform oscillates at the depth of 2 μm, no oscillation occurs at 30 μm.
Creating a deep n+-type buffer layer with a thickness of around 30 μm by diffusion of phosphorus takes 24 hours or more at a general heat treatment temperature such as 1100° C. and mass productivity is low. Other methods include using, an accelerator such as a cyclotron or a Van de Graaff generator (for example, refer to PTL 1). For example, irradiating a silicon substrate with protons at an accelerating voltage of 8 MeV yields a range of approximately 480 μm and a half-value width of approximately 20 μm. By driving protons through an absorber, instead of driving the same directly into the silicon substrate in order to adjust a position of a range, irradiation energy can be decelerated and a broad proton peak can be created near a surface of silicon. Subsequently, by performing heat treatment of 1 to 5 hours at 350° C. to 450° C., protons are activated and an n-type region can be formed. Moreover, although also dependent on implantation conditions and heat treatment conditions, an activation rate of protons is around 1%.